I tried a -current domU with XenServer and in conjunction with a
patch to enable /kern/xen/xenbus, I fed XenServer all the right info
for it to accept NetBSD as a supported guest (thus enabling migrate,
suspend, memory ballooning, etc.). When I tried to migrate, it sent
through a sleep button press on xenbus which I guess would need to be
caught by powerd to action a suspend within the domU. Is there
something within the NetBSD domU to do this?

Yes; I made a modification to -current where powerd(8) is "enabled" by
default when you are in the following situation: